It seems you've mixed up semi-major axis (a in the equation) with periapsis (r) - they are not interchangeable except in the case of a circular orbit. The semi-major axis for a hyperbolic orbit is negative, not infinite. We would need to know some other information about the asteroid's initial orbit to obtain the semi-major axis - eccentricity for example. Though, you did correctly set the lower bound for the asteroid's velocity using the escape velocity equation, calculated at the asteroid's periapsis. I'm still trying to figure this problem out - it's got me stumped.
And, just a nitpick - the equation for kinetic energy comes from Newton's first law (F = m*dv/dt) and some calculus, not the third law.